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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail2) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Ambac UK [Member]
Investment [Line Items]
Asset-backed securities fair value	$ 144,672	$ 170,280

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- Definition

Asset-backed securities guaranteed by Ambac UK.

+ References

No definition available.

+ Details

Name:	ambc_AssetBackedSecuritiesFairValueDisclosure
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Definition

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	dei_LegalEntityAxis=ambc_AmbacUkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: